1. Field of the Invention
The present invention relates to a differential gear device incorporating an internal meshing type planetary reduction gear having an internally toothed gear composed of pins or a combination of pins and rollers, and an externally toothed gear having a trochoidal or arcuate tooth form.
2. Description of the Prior Art
A differential gear is used, for example, in the driving system of a vehicle for the purpose of driving left and right driving wheels of the vehicle in such a manner as to absorb difference in the rotation speed between both driving wheels when the vehicle turns around a corner.
FIG. 4 illustrates an intersecting axis type differential gear device which is the most popular one. This differential gear device has a drive shaft 3 the axis of which intersects the axes of the left and right axles 1 and 2 which extend to the left and right.
FIG. 5 shows a parallel axis type differential gear device which is adopted in many vehicles of FF (front engine front drive) type. In this type of differential gear device, a drive shaft 3 extends in parallel with left and right axles 1 and 2 which extend to the left and right.
A two-staged differential gear device of the parallel axis type, specifically intended for use in four-wheel driving vehicles, is also known in which a primary differential gear device and a secondary differential gear device are connected in series, as disclosed in Japanese Utility Model Unexamined Publication No. 61-188065. In this type of differential gear device, the rotation of the output shaft of the transmission is transmitted from an output gear on the output shaft to a ring gear provided on the gear box of the primary differential gear device, and the rotation speed is primarily reduced due to the difference in the number of gear teeth of the output gear and the ring gear. In addition, one of the rotary shafts of the primary differential gear device is hollow, while the other serves as the gearbox of the secondary differential gear device. One of the rotary shafts of the secondary gear device extends through this hollow rotary shaft.
The following problems are encountered with these known differential gear devices.
Referring first to the intersecting axis type differential gear device shown in FIG. 4, the device inevitably has a large size because the axis of the drive shaft 3 substantially orthogonally intersects the axes of the left and right axles 1 and 2. In addition, it is difficult to obtain a large reduction ratio with this type of differential gear device.
The differential gear device of parallel axis type shown in FIG. 5 can have a more compact design but the reduction in the size is still unsatisfactory. This type of differential gear device cannot provide large reduction ratio.
The differential gear device of the type disclosed in Japanese Utility Model Unexamined Publication No. 61-188065 is designed specifically for four-wheel driving vehicles such that one of the rotary gears of the primary differential gear device is made hollow to allow one of the rotary shafts of the secondary differential gear device to extend therethrough. The primary differential gear device is not intended for use as a reduction gear, though the speed is reduced primarily when the rotation is transmitted from the output gear of the transmission to the primary differential gear device. With this type of differential gear device, it is difficult to obtain a compact design and large reduction ratio, as in the case of the differential gear device of the parallel axis type.